One of the first olefinically unsaturated synthetic elastomers to be commercially produced was butyl rubber. The expression "butyl rubber" is used in the rubber industry to describe copolymers made from a polymerization reaction mixture having therein from 70 to 99.5% by weight of an isoolefin which has about 4 to 7 carbon atoms, e.g. isobutylene, and about 30 to 0.5% by weight of a conjugated multiolefin having from 4 to 14 carbon atoms, e.g. isoprene. The resulting copolymers contain 85 to 99.5% by weight combined isoolefin and about 0.5 to about 15% combined multiolefin.
The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128, which is incorporated herein by reference. Butyl rubber generally has a number average molecular weight of about 5,000 to about 500,000, preferably 80,000 to 250,000 and a Wijs Iodine No. of about 0.5 to 50 preferably 1 to 15. Low molecular weight butyl rubber is generally defined as having a M.sub.v of 5,000 to 30,000 and 2-10 mole % unsaturation. The viscosity average molecular weight (M.sub.v) of commercial butyl rubber is about 100,000 to about 500,000, preferably about 250,000 to 500,000.
The polymer backbone of commercial butyl rubber is made up primarily of isobutylene units, with just a few percent isoprene units. The isoprene units contribute the small amount of unsaturation present in butyl rubber. The basic preparative equations are represented by: ##STR1## which combine to form its main structure: ##STR2## wherein n+l represents the number of isoolefin units incorporated in the butyl rubber, while m represents the number of diolefin units present, substantially as isolated units. The conjugated diolefin loses its diene unsaturation upon its incorporation into the polymer backbone.
Thus butyl rubber, as presently produced, contains only a small percentage of unsaturation, in the form of the monoolefin structure associated with the isoprene residue which is incorporated more or less randomly throughout the polymer chain.
The reactivity of the butyl rubbers and consequently their cure rate is substantially less than the high unsaturation natural and synthetic rubbers. In an effort to improve cure characteristics of the butyl rubbers, these synthetic polymers have been halogenated. Halogenated butyl rubber has contributed significantly to the elastomer industry. A method of preparing halogenated butyl rubber is described in U.S. Pat. No. 3,099,644 which is incorporated herein by reference. Both chlorinated and brominated butyl rubbers are known in the art. The structural formula for halogenated butyl rubber can be represented as being: ##STR3## where X represents the halogen and n, l and m have the same values as described above for butyl rubber. This structure, however, is one of several which can be formed, depending on the conditions of halogenation, the halogenating agent, used etc. The primary structural configurations generally found in halogenated butyl rubbers are ##STR4## Other structural configurations which may occur are: ##STR5## It will be noted that in each case the halogen is present as a secondary or tertiary allylic halogen.
Methods for the halogenation of butyl rubber are well known in the art. U.S. Pat. No. 2,631,984 discloses various techniques for the bromination of butyl rubber. Where solid brominating agents such as N-bromine succinimide is utilized in the solid phase bromination of butyl rubber the mill temperature can be as high as 145.degree. C. However, the solution bromination utilizing bromine as the brominating agent is disclosed as being carried out in ethyl chloride at temperatures at which ethyl chloride (b.p. 12.5.degree. C.) is a liquid.
U.S. Pat. No. 2,732,354 discloses a method for the chlorination of butyl rubber using a chlorinating agent at a temperature below 0.degree. C., e.g., 31 20.degree. C. to about -65.degree. C.
U.S. Pat. No. 2,965,620 discloses a process for the preparation of halogenated butyl rubber by adding a minor amount of water to a butyl rubber solution in order to produce a suspension of the butyl rubber solution and halogenating the rubber. While a broad temperature range of 0.degree. C. to 30 150.degree. C. is disclosed for the reaction, the preferred temperature range is disclosed to be 20.degree. C. to 50.degree. C. The examples disclose bromination carried out at room temperature to about 80.degree. C. U.S. Pat. No. 2,965,621 discloses a similar process wherein metal salts are dissolved in the water. While similar temperatures are disclosed for the halogenation process, the examples were all conducted at room temperature.
U.S. Pat. No. 3,009,904 discloses a method for preparing a chlorinated/brominated butyl rubber which comprises reacting a solution of the rubber, sequentially, first with chlorine, then with bromine. While broad disclosures of useful temperatures are made, e.g., 0.degree. to 200.degree. C., the preferred range of reaction temperatures is taught to be 20.degree. C. to 50.degree. C. In fact all of the examples show the bromination of a previously chlorinated butyl rubber at room temperature.
A process for the chlorination of butyl rubber dissolved in an inert solvent is disclosed in U.S. Pat. 2,964,489. The reaction temperature range is taught to be 0.degree. C. to about +100.degree. C. preferably 10.degree. C. to 80.degree. C. The more preferred and claimed range, is disclosed to be 20.degree. C. to 70.degree. C. The examples, however, were conducted at about 30.degree. C.
U.S. Pat. No. 3,099,644 discloses a continuous chlorination process which is disclosed to be operable at about 0.degree. to 100.degree. C., preferrably at about 20.degree. C. to 80.degree. C. While bromination is alluded to, no conditions for the bromination is taught.
British Patent Specification No. 1,008,656 published Nov. 3, 1965 discloses that where elemental bromine is used to prepare brominated butyl rubber the reaction must be carried out below 0.degree. C. An improved bromination process is taught using brominated aliphaticdiamine as the brominating agent at temperatures of less than 50.degree. C. The art has not generally recognized any advantage to carrying out the bromination reaction at any particular temperature.
More recently, U.S. Pat. No. 4,288,575 to Irwin Gardner (which has an effective filing date of Mar. 7, 1977) discloses a new structural configuration for the halogenated rubber where the rubber contains conjugated diene which is represented as ##STR6##
In this structure the halogen, X, is in a primary allylic position. The method disclosed in U.S. Pat. No. 4,288,575 for preparing these rubbers involves the use of a copper oxide catalyst useful for dehydrohalogenation of butyl rubber to form a conjugated diene rubber.
As shown in Example 6 of the Gardner U.S. Pat. No. 4,288,575 this primary halogen is in a more stable configuration than the secondary halogens of the prior art and is not readily removed. The copper oxide catalyst was taught in Gardner's earlier U.S. Pat. No. 4,145,492 to be a dehydrohalogenation catalyst suitable for the preparation of conjugated diene rubber. Where Gardner produces polymers containing the structure of Formula V that structure is invariably associated with conjugated diene.
Table I of U.S. Pat. No. 4,288,575 shows various halogenated conjugated diene-containing polymers which are shown to have the halogen in the primary position. Not surprisingly, the residual halogen is always associated with substantial amounts of conjugated diene. Since the catalyst is a dehydrohalogenation catalyst, dehydrohalogenation is proportional to the degree of contact of polymer with the catalyst, and similarly, the degree of rearrangement of halogen from the secondary to the primary position is related to the degree of dehydrohalogenation.
Where high amounts of residual halogen are present in the polymer, as in Run A of Table I of U.S. Pat. No. 4,288,575, it is the result of an initially high level of halogenation; here 1.95 wt. % bromine. Since the degree of rearrangement is proportional to the degree of dehydrohalogenation Gardner's polymers cannot be low in conjugated diene and at the same time have appreciable amounts of halogen present in the primary allylic position.
In 1979 Van Tongerloo et al. disclosed a brominated butyl rubber which was low in conjugated diene content (if any) and had the primary halogen configuration. The polymer is represented as having the structure ##STR7## The reference states that the polymer was produced by a proprietary method and Van Tongerloo et. al. disclose only that rearrangement to the more stable primary configuration can be accomplished in brominated butyl rubber "under a variety of conditions--for example, in the presence of acid, free radicals, bases or heat." See Van Tongerloo, A. and Vukov, R., Proceedings, International Rubber Conference, Milan, Italy, 1979, p.70ff. The skilled chemist will recognize that this gratuitous disclosure represents the techniques which can be enumerated to accomplish an infinite number of reactions. The disclosure in no way teaches any method to prepare the polymer disclosed.
Van Tongerloo et. al. designate brominated butyl rubber having the secondary allylic bromine associated with the methylene configuration of Formula VIII above as "EXO" and the primary bromo configuration of Formula VII as "ENDO". It is alleged that even at ratios of ENDO:EXO of 71:16 there is no clear indication of a correlation between vulcanizate properties and polymer microstructure. Hence, Van Tongerloo et. al. have not appreciated that the polymer, which they purportedly made by an undisclosed proprietary process, has any properties which are different than those of conventional halogenated butyl rubber.
In a later paper Vukov disclosed that certain model compounds can be heated to 150.degree. C. for 30 minutes to accomplish a molecular rearrangement as follows: ##STR8## No substantial rearrangement of the chlorinated model was observed. See Vukov, R., "Halogenation of Butyl Rubber and The Zinc Oxide Cross-Linking Chemistry of Halogenated Derivatives" which was presented to the ACS Rubber Division on Oct. 25, 1983. Those skilled in the art will recognize that what is true about simple molecules (model compounds) may not necessarily be true about complex polymer molecules.
While the art generally teaches a broad temperature range for the production of brominated butyl rubber in practice elevated temperatures are avoided by those skilled in the art. Commercially, bromination is carried out at room temperature. While occassional temperature excursions as high as 50.degree.-55.degree. C. occur they are avoided because they have been believed to result in poor quality product. Whenever the art has attempted to produce a brominated butyl rubber having a substantial fraction of its bromine in the primary allyic position, this result has been achieved by an isomerization reaction on previously brominated butyl rubber. No direct process for producing such a product has been disclosed or suggested in the art.